Ornamental · Apocynaceae
Blue Star
Amsonia tabernaemontana
Give it full sun and a trim after bloom, and Blue Star takes you from spring flowers to gold fall leaves without much fuss at all.
Full sun
Every 7–14 days
USDA 3–9
2–3 ft · 0.6–0.9 m
Overview
Want a perennial that mostly looks after itself? That is Blue Star. It is a tough native that grows slowly into a shrubby clump of feathery green leaves, and in late spring it covers itself in clusters of soft, steel-blue star-shaped flowers. It is part of the dogbane family, and once it is rooted in it just shrugs off cold, heat, clay, and dry spells, so you do not have to baby it. Hardy from zone 3 to zone 9, it will grow just about anywhere you put it.
And the flowers aren't even the whole story. When fall rolls around, those narrow leaves turn a clear golden yellow that really brightens up a border, and the bitter, milky sap inside keeps deer and rabbits from snacking on it. Plant it in full sun and it stands up on its own without any staking, so think of it as that dependable, no-drama plant you can build the rest of the bed around.
Care
The short version: full sun, average well-drained soil, and a good hard trim once the flowers are done. It barely needs watering after it settles in and hardly any feeding, so honestly the only regular thing you have to do is cut it back so it doesn't flop.
Light
Blue Star really wants full sun, and that is where it grows nice and dense and holds itself up without staking. It will put up with partial shade or partial sun, but the less light it gets, the more it tends to stretch out, lean, and flop.
Water
While it is getting settled, keep the soil evenly moist and water it every 7 to 14 days. After the roots take hold, though, it shrugs off dry spells and even puts up with the odd soggy patch, so you can pretty much back off the hose.
Soil
It really isn't picky about soil and will grow in loam, clay, or sand as long as the water drains. Anything from pH 6.0 to 8.0 keeps it happy, and that covers just about any garden dirt you have got.
Temperature
This has to be one of the toughest perennials around, taking winters all the way down to -40°F (-40°C) and summer heat up to 100°F (38°C) without blinking. That is the whole reason you can count on it from zone 3 right through zone 9.
Humidity
Regular outdoor air is all it is after, so there is no humidity number to worry about. What matters more is good airflow, because crowded, stuffy spots are exactly what let rust get a foothold on the leaves.
Feeding
Feeding? You can mostly skip it. A light topdressing of compost in spring is plenty, and loading it up with rich soil or fertilizer just makes the stems soft and floppy.
Pruning
The one thing that really counts is cutting the stems back by one-third to one-half right after the flowers fade. That keeps the clump rounded and self-supporting, and if you deadhead at the same time, it won't go seeding itself all over the garden.
Repotting
Settled clumps almost never need lifting, and honestly they would rather you left them be. Only dig and divide when the center starts to die out or the flowering tails off, and that can be years down the road.
Size & spacing
A grown plant gets to about 2 to 3 feet (61 to 91 cm) tall and spreads roughly the same. Give each one around 2 to 3 feet (61 to 91 cm) of space so the air can move through and rust stays away.
When to plant
Blue Star is a hardy perennial, so it comes back on its own every year without any replanting. Pop new plants in during spring after the frost or in fall, keep an eye out for those blue flowers in late spring, and then just cut the whole clump back hard once it is finished blooming.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Late winter to early spring; seed germinates readily but plants typically flower in their second year.
- Plant: Spring after frost, or fall.
- Harvest: Late spring (April to May), into early summer.
Hardiness
USDA hardiness zones: 3–9
Temperature: down to -40°C
Troubleshooting
Here is the good news: Blue Star almost never gives you trouble, and that is a big part of why people love it. There are really only two things you might bump into, floppy stems and a bit of rust now and then. Find what looks familiar below, and the fix is usually pretty quick.
Stems that splay open and flop outward
If your plant is splaying out, flopping usually means too much shade or soil that is a bit too rich, and it gets worse when you skip the trim after it blooms. Shear the stems by a third to a half once the flowers fade and you will get a tighter mound that holds itself up. And if the clump is sitting in shade? Move it somewhere sunnier and it will stand on its own.
Fix:
- Cut stems back by one-third to one-half after flowering to encourage a denser, self-supporting habit
- Relocate to a sunnier position if grown in shade
Orange, rusty bumps on the leaves
Powdery orange-to-brown spots on the undersides of the leaves usually mean rust, which is about the only disease this plant picks up. Pull off the spotted foliage and toss it in the bin. Then open the clump up a little and keep water off the leaves, since rust loves still, damp air.
Fix:
- Remove and dispose of affected foliage
- Improve air circulation and avoid overhead watering
Toxicity
Blue Star is about as low-risk as a garden plant gets. That bitter, milky sap keeps deer, rabbits, and most nosy pets from bothering with it, and it isn't on the ASPCA toxic list either. The one thing to keep in mind? The latex from cut stems can irritate skin and eyes, so just wash your hands after you prune.
Toxic to
Cats · Dogs · Horses · Livestock · Birds · People
Safe for
Rabbits
Cats Caution
There isn't a report written up specifically for cats, but that bitter milky sap comes from the same family as other irritant plants. So if a curious cat chews a leaf, it might drool or get a quick stomach upset, though the taste usually sends it packing after one bite.
Dogs Mild
One extension source calls the milky sap slightly toxic to dogs, so a chewed stem can leave a dog with mild mouth or stomach irritation. The good thing is that bitter latex usually stops most dogs before they swallow much of anything.
Horses Caution
There is nothing specific on record for horses, but the bitter sap and that dogbane background make it one to keep them away from. Most horses leave it alone on their own anyway.
Livestock Caution
Grazing animals turn up their noses at the bitter foliage, so trouble is unlikely. Still, it is smart to keep the milky latex out of pasture where hungry stock don't have much else to eat.
Birds Caution
There is no record of it harming birds, and they aren't drawn to the foliage anyway. The only time to watch is if you have got a pet bird that likes to nibble houseplants and cut greenery.
People Caution
No part of this plant is meant to be eaten. The milky latex from cut stems can irritate skin or eyes, so wash your hands after pruning and keep small children from chewing on the foliage.
Rabbits Safe
Rabbits steer well clear of this one. The bitter taste puts them off so reliably that gardeners actually plant it where rabbits are a headache, and browsing damage is pretty rare.
The milky latex released when stems or leaves are cut or torn can be a mild skin or eye irritant; wash hands after pruning.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
A handful of named picks tweak the wild plant a bit, usually for richer flower color or showier leaves. These are the ones you will actually run across at most nurseries.
Storm Cloud
With Storm Cloud, the stems come up nearly black in spring, which looks great against dark green leaves streaked with bright silver veins.
Blue Ice
Blue Ice is a compact one, and its flowers are a deeper, more violet-blue than you get on the plain species.
var. salicifolia
Then there is var. salicifolia, which has narrow, willow-like leaves only about a quarter inch wide.
How to propagate
Blue Star takes its time spreading, so if you want more of it, you will probably want to give it a hand. You can split the clumps, save some seed, or root a few cuttings, whichever sounds easiest.
DivisionEasy
Spring or fall while dormant · One growing season
The simplest way is to lift and split the clump while it is dormant in spring or fall, just making sure each piece keeps at least one eye, or growth bud. The divisions usually settle in over a single growing season, so you won't wait long.
SeedEasy
Late winter to spring · Flowers usually in the second year
You can sow seed from late winter into spring and it sprouts without any fuss, though the young plants tend to hold off blooming until their second year. Snip off any seedheads you do not want, otherwise the plant will sow itself around.
Stem cuttingsModerate
Late spring to early summer · Several weeks to root
If you are feeling up for it, take root or stem cuttings from non-flowering shoots in late spring or early summer. They take several weeks to root, which makes this the slower and fiddlier option next to just dividing.
Myths
Blue Star is dangerously poisonous to people and pets and should be kept out of the garden.
Not really, though. Just one extension source flags the milky sap as slightly toxic to dogs, and the bitter taste keeps most animals from eating it in the first place. It is widely grown as a safe, low-maintenance native ornamental, and you won't find it on the ASPCA toxic plant list.